Dileptons as a Probe of QGP

Dileptons as a Probe of QGP
See :Ch. Wong Introduction to High Energy Heavy Ion Collisions Ch.14 R. Vogt Ch. 7 The contributions to final spectra could be 1. from QGP phase qq-> l+l- 2. from mixed phase: qq and hadrons -> l+l- 3. from hadron gas phase At high T for masses above φ (1.004 GeV) and below J/PSI (3 GeV) there is a gap for thermal dilepton l+l- spectra from QGP. How to calculate it and to measure it? Do we see any excess of dileptons in dN/dM mass spectra in the experiment?

γ g* ℓ + Why do we measure them ℓ -
Convey dual information convoluted over the history of the collisions: probe surrounding matter properties: T, density, flow… probes themselves are affected by hot and dense matter ℓ + ℓ - γ g* Modification of Vector Meson spectral functions close to Chiral Symmetry Restoration point: VM broadening vs mass shift. Thermal emission: both from QGP and Hadronic phase Rich variety of dilepton contributions in hadronic phase:  Low Mass Range (LMR, m ≤ m ) : resonant (VM, mostly r);  Intermediate Mass Range (IMR, m <m < mJ/ ) : continuum-like; rates determined by T (parton–hadron duality?)  need to disentangle experimentally QQP and HG contributions. Strangeness () enhancement from abundantly produced in QGP Quarkonia suppression by colour screening in QGP (covered by R.Arnaldi talk) Caveats: experimental: small yields, strong backgrounds … interpretational: underlying ‘conventional’ sources need to be understood …

Lepton-Pair Continuum Physics
Modifications due to QCD phase transition Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation & modified heavy flavor suppression (enhancement) known sources of lepton pairs at s = 200 GeV Sources “long” after collision: p0, h, w Dalitz decays (r), w, f, J/y, y‘ decays Early in collision (hard probes): Heavy flavor production Drell Yan, direct radiation Baseline from p-p Thermal (blackbody) radiation in dileptons and photons temperature evolution Medium modifications of meson pp  r  l+l- chiral symmetry restoration Medium effects on hard probes Heavy flavor energy loss Large discovery potential also RHIC Axel Drees

Dileptons and direct photons at SPS
R. Shahoyan, CERN Motivation and milestones Excess in dilepton production f-puzzle Photons QUARK MATTER 2009, March 30 – April 4, Knoxville, TN

γ g* ℓ + Why do we measure them ℓ -
Convey dual information convoluted over the history of the collisions: probe surrounding matter properties: T, density, flow… probes themselves are affected by hot and dense matter ℓ + ℓ - γ g* Modification of Vector Meson spectral functions close to Chiral Symmetry Restoration point: VM broadening vs mass shift. Thermal emission: both from QGP and Hadronic phase Rich variety of dilepton contributions in hadronic phase:  Low Mass Range (LMR, m ≤ m ) : resonant (VM, mostly r);  Intermediate Mass Range (IMR, m <m < mJ/ ) : continuum-like; rates determined by T (parton–hadron duality?)  need to disentangle experimentally QQP and HG contributions. Strangeness () enhancement from abundantly produced in QGP Quarkonia suppression by colour screening in QGP (covered by R.Arnaldi talk) Caveats: experimental: small yields, strong backgrounds … interpretational: underlying ‘conventional’ sources need to be understood …

Milestones I Nucl.Phys.A590 1995) 93c Eur.Phys.C13(2000) 433
Helios-3 p-W, 200 A GeV : LMR/IMR enhancement Thermal emission? Strong pa1 mm contribution at IMR Li, Gale, Phys.Rev.Lett. 81(1998) 1572 Helios-3 200 A GeV/c NA38 p-W, S-U @ 200 A GeV NA50 p- 450 GeV Pb-Pb @ 158 A GeV IMR in p-A described by Drell-Yan and Open Charm Yields in Pb-Pb exceed the extrapolation from the p-A IMR excess resembles (m, pT) : nuclear modifications? Also described by thermal emission from GQP and HG Rapp and Shuryak, Phys. Lett. B473 (2000) 13 NA38/NA50 pA 450 GeV/c central collisions NA50 Pb-Pb 158 A GeV/c <Npart> = 381 Eur.Phys.C14(2000) 442

Milestones II / CERES measurements
coils TPC 1999/2000 only Two RICH detectors separated by solenoidal field 2 SiDC in vertex region trigger on multiplicity p-Be,Au 450 GeV : good descripiton by hadronic ‘cocktail’ decays Central S-Au 200 A GeV excess: ± 0.7(stat) ± 2.0(syst) Eur.Phys.J.C 4 (1998) 231 Phys.Rev.Lett.75(1995 )1272 >400 citatio ns

LMR excess by CERES / S-Au 200 A GeV
Brown/Rho Rapp/Wambach Vacuum r Not described by pp annihilation in vacuum r mass drop (Brown/Rho scaling): Li, Ko, Brown, Phys.Rev.Lett.75 (1995) 4007 r broadening: Chanfray, Rapp, Wambach, Phys.Rev.Lett. 76 (1996) 368 23

LMR excess by CERES / Pb-Au 158 A GeV
Brown-Rho scaling Rapp-Wambach Vacuum-r from pp-annihilation Eur.Phys.J. C 41 (2005) 475 Excess seen in S-Au confirmed in Pb-Au 158 A GeV ±0.25(stat) ±0.65(syst) ± 0.82(decays) 1999 TPC upgrade: sM/M 6%  4% at / region 2000 data Phys. Lett. B666 (2008) 425 RW BR Excess: 2.61 ± 0.31(stat) ± 0.43(syst) ± 0.76(decays) Phys.Rev.Lett. 91 (2003) RW BR 1999 data The only dilepton data at 40 A GeV higher excess: 5.9±1.5(stat)±1.2(syst)±1.8(decays) Effect of higher baryonic density?

Measuring dimuons in NA60
2.5 T dipole magnet hadron absorber targets beam tracker Si-pixel tracker muon trigger and tracking (NA50) magnetic field >10m <1m Phys.Rev.Lett. 96 (2006) Radiation-hard silicon pixel vertex tracker Muons from the NA50 spectrometer matched to tracks in the vertex region: Improved mass resolution  offset wrt the vertex ( @ ~20 GeV)  promt vs open charm (ct=123,312 mm) separation Dipole improves low-mass and low-pT acceptance Matching rejects decay kinks  improved S/B Selective trigger, high luminosity

NA60 LMR data: peripheral (Nch<30) In-In collisions
Well described by meson decay ‘cocktail’: η, η’, ρ, ω, f and contributions (Genesis generator developed within CERES and adapted for dimuons by NA60). Eur.Phys.J.C 49 (2007) 235 Eur.Phys.J.C 43 (2005) 407 Similar cocktail describes NA60 p-Be,In,Pb 400 GeV data

EM transition form-factors for and
EM transition form-factors for and peripheral NA60 InIn data (hep-ph/ , submitted to PLB) Acceptance-corrected data (after subtraction of , and  peaks) fitted by three contributions: pole approximation: Confirmed anomaly ofF wrt the VDM prediction. Improved errors wrt the Lepton-G results. Removes FF ambiguity in the ‘cocktail’ In-In, peripheral

More central In-In data
Clear excess of data above decay ‘cocktail’ describing peripheral events Phys. Rev. Lett. 96 (2006) Excess isolated subtracting the measured decay cocktail (without r), independently for each centrality bin Based solely on local criteria for the major sources: η, ω and f  2-3% accuracy.  No need of reference data (pA, peripheral data, models)  Less uncertainties (e.g. strangeness enhancement: ,)

NA60 LMR Excess dimuons Eur.Phys.J.C 49 (2007) 235 Excess above the cocktail ρ (bound by ρ/ω=1.0), centered at nominal ρ pole Monotonically rises and broadens with centrality By coincidence, NA60 acceptance roughly removes the phase-space factor  r spectral function convoluted over the fireball evolution is directly measured

Centrality dependence of LMR excess
NA60, In-In 158A GeV CERES, Pb-Au 158A GeV 95/96 data combined data/cocktail r Eur.Phys.J. C41 (2005) 475 peak: R=C-1/2(L+U) continuum: 3/2(L+U) cocktail ρ is fixed by ρ/ω=1.0 Excess rises faster than linear with multiplicity: compatible with emission from annihilation processes Total excess wrt “cocktail” r: roughly indicative of the number of rho generations: r – clock? Question to theory: can the fireball life-time be inferred? How does the modified r life-time evolve? Eur.Phys.J.C 49 (2007) 235

LMR Excess: r dropping mass vs broadening
Phys. Rev. Lett. 96 (2006) Phys. Lett. B666 (2008) 425 NA60, InIn 158A GeV CERES, Pb-Au 158A GeV cocktail subtracted using stat.model Calculations by R.Rapp for both scenarios Only broadening of r observed; interactions with baryons is very important Mass shift (Brown/Rho scaling) is ruled out. (CERES data also described by flat spectral function: Kämpfer et all, Nucl. Phys. A 688 (2001) 939)

NA60 IMR data (1.16 < M < 2.56 GeV/c2)
Eur.Phys.J. C59 (2009) 607 Fit range Mass spectrum is similar to NA50: Good description by Drell-Yan + ~2Open Charm (extrapolated from pA data) Prompt ~50mm ~1mm Such explanation is rejected by the spectra of dimuon offsets wrt the interaction vertex! 1.120.17 Data Prompt: 0.08 Charm: 0.16 Fit 2/NDF: 0.6 Prompt Offset fit shows that the enhancement is not due to Open Charm  the excess is prompt 32

NA60 IMR excess (1.16 < M < 2.56 GeV/c2)
Mass (GeV/c2) Mass shape and yield close to Open Charm contribution measured agrees within ~20% with from NA50 pA data (same kinematical domain |cosCS|<0.5)  no strong modifications. Scales with centrality faster than Drell-Yan (~Nbin.coll), but less faster than Eur.Phys.J. C59 (2009) 607 Much softer in pT than Drell-Yan: rules out higher-twist DY? [Qiu, Zhang, Phys. Lett. B 525, (2002) 265]

NA60 excess: comparison to theory
Eur.Phys.J. C59 (2009) 607 All known sources subtracted (‘cocktail’, Drell-Yan, Open Charm) Corrected for acceptance Integrated over pT Absolute normalization: data and models. H. van Hees and R. Rapp, Nucl. Phys. A 806 (2008) 339 : mostly pp annihilation in LMR (collisional broadening of r, strong effect from baryons), 4p in IMR (full chiral mixing  enhanced a1), QGP contribution 20 – 60% (fireball scenario A … C) T. Renk and J. Ruppert, Phys. Rev. C 77, (2008) : mostly pp annihilation in LMR (r spectral function by Eletsky et al, Phys.Rev.C 69 (2001) ), QGP dominates (~80%) in IMR K. Dusling, D. Teaney and I. Zahed, Phys. Rev. C 75, (2007) PhD thesis of K.Dusling : hydrodynamic calculation with virial expansion of hadronic rates, QGP contribution dominates in IMR: 60 – 90%

NA60 excess vs pT: comparison to theory
J.Phys. G 35 (2008) Absolute normalization both for theory and data Differences at low masses reflect differences in the tail of r spectral function Differences at high masses, pT reflect differences in flow strength

mT spectra of NA60: excess
Eur.Phys.J. C (2009), in press, nucl-ex/ ‘Cocktail’ r and excess continuum are separated using side-window method: r is hotter M Teff 0.2 – ± ± ±4 f 242± ±11 (DY not subtracted) Phys. Rev. Lett. 100 (2008) Fit of excess by (‘cocktail’ r is not subtracted) Teff rises up to the f mass, then drops Spectra steepen at low mT (not for hadrons) Confirmed by independent IMR excess analysis Fit in 0.5<PT<2 GeV/c 36

mT spectra from NA60 Hierarchy of hadrons freeze-out
J.Phys. G 35 (2008) Blast wave analysis of NA60 data: crossing of hadrons with p defines Tf, and bT max reached at respective hadron freeze-out Suggests different freeze-out time for different hadrons: f first, r last (maximal coupling to pions) Large difference between r and w

Teff evolution with mass
Eur.Phys.J. C (2009), in press, nucl-ex/ LMR Thermal emission dominates in HG phase (RH, RR, DZ models agree) Integration over the flow development  rise of Teff with mass (RH: relatively week flow compensated by primordial contributions, but insufficient for pure in-medium LMR emission) Hadrons affected by different freeze-out times <> IMR RR, DZ: thermal emission dominates in the QGP phase, when flow has not yet built up  Teff are not affected by blue shift  no strong mass dependence RH : emission from HG dominates

Polarization of dileptons
Submitted to PRL, nucl-ex/ NA60 also measured the polarization (in the Collins-Soper frame) for m≤ m Lack of any polarization in excess (and in hadrons) supports emission from thermalized source. Details in the presentation of G.Usai /session 4C/

Evidence of ω in-medium effects?
Eur.Phys.J. C (2009), in press, nucl-ex/ nucl-ex/ Flattening of the pT distributions at low pT, developing very fast with centrality. Low-pT ω’s have more chances to decay inside the fireball ? Appearance of that yield elsewhere in the spectrum, due to ω mass shift and/or broadening, unmeasurable due to masking by the much stronger ppmm contribution. Disappearance of yield out of narrow ω peak in nominal pole position  Can only measure disappearance

NA60 results on omega yield suppression
Determine suppression vs pT with respect to (extrapolated from pT>1GeV/c) Account for difference in flow effects using the results of the Blast Wave analysis Eur.Phys.J. C (2009), in press nucl-ex/ Reference line: f/Npart = f.ph.s. (central coll.) Consistent with radial flow effects Reference line: ω/Npart = f.ph.s. Strong centrality-dependent suppression at pT<0.8 GeV/c , beyond flow effects

f - puzzle Disagreement between results for f in Pb-Pb 158A GeV
NA49 fK+K- (pT<1.6 GeV/c) NA50 fm+m- (pT>1.1GeV/c) [Phys. Lett. B491, (2000) 59; J. Phys. G 27,(2001) 355] [Nucl. Phys. A661 (1999)534c; Phys. Lett. B 555 (2003) 147] NA50 sees >2 times higher yield than NA49 Large difference in Teff : Tmm =218±6 TKK = 305±15 In-medium effects on f and K + K absorption and rescattering  reduced yield and harder pT spectrum in hadronic channel? Recent developments: CERES mesured fK+K- and fe+e- channels in central Pb-Au 158A GeV [Phys.Rev.Lett. 96 (2006) ]  Both channels agree with each other (large errors on e+e- channel) and with NA49 NA50 reanalysed its old Pb-Pb data [J. Phys. G 35 (2008) ]  Old results are confirmed within 8% New results from NA60 in In-In 158 A GeV data : comparison of fm+m- and fK+K-

f - puzzle NA60 finds good agreement between the m+m- and K+K- channels  no f – puzzle in In-In Teff in central In-In collisions is lower compared to cetral Pb-Pb of NA49 and CERES <f>/Npart in central In-In collisions is lower than the NA50 results (f.ph.s), but slightly higher than measured by NA49 and CERES Details in the presentation of A.de Falco /session 5D/

Direct photon (old) measurements at SPS
Extremely difficult to measure: large background from p0 and ’  very few results (especially at low pT where QCD contribution is small) 15-20% upper limits (90%CL) wrt the hadronic sources for central S-Au 200 A GeV/c from CERES [Z.Phys.C71 (1996) 571] and WA80 [Phys.Rev.Lett. 76 (1996) 3506] Most significant results from WA98 in central Pb-Pb 158 A GeV [Phys.Rev.Lett.85 (2000) 3595]: excess of up to 20±7% for pT>1.5 GeV/c Calculation by Turbide, Rapp and Gale within the same approach as for l+l- Phys.Rev.C69 (2004) Even more difficult to interpret than in the case of dileptons: same ambiguity: close to the Tc hadronic and partonic descriptions provide similar rates… no extra handle like mass, only pT

Direct photons from internal conversion ?
PHENIX: interpret enhancement at pT>>mee as internally converted direct photons (‘almost’ real: 0.1<mee<0.3 GeV/c2, pT>1 GeV/c)  Teff = 2212318 MeV interpret as TQGP (weighted over the QGP evolution)  Ti = 300 – 600 MeV Teff = 183  10 MeV Ti = 210–350 MeV ? NA60 observes an excess at same masses Smoothly continues to m>0.3 GeV/c2 (in PHENIX also) Exponential down to low pT’s (‘high’ virtuality) Teff = 183 MeV fits well to the smooth rise with M (flow)  Tth ~ 160 MeV No evidence of direct ‘real’ photons ?

Prospects from LHC? Much higher Tinitial and life-time of the fireball  stronger thermal emission of , l+l- (Surprisingly, PHENIX sees excess only at M < 750 MeV/c2, see next presentation by A.Drees) Even higher background to cope with: Sll /B < vs 10-1 on SPS ALICE, ATLAS, CMS: excellent capabilities to measure photons (ALICE and CMS from pT > ~1 GeV/c) See presentation of D. Peressounko on photons in Alice /session 5D/ Possibilities of l+l- measurements below J/ are limited by acceptance, tracking, PID Currently, only ALICE has some preliminary estimates for low mass dileptons: +- : no acceptance below mT ~ 1 GeV/c due to the 0.5 GeV/c cut on single m pT e+e- : good PID using TPC+TRD for pT > 1GeV/c Expectations from Alice Muon Arm 1 month of Pb-Pb

Summary Excess in dilepton emission at SPS: good agreement with models of thermal emission ‘Planck’-like mass spectra (slightly modified by non-flat spectral function in LMR) Teff rising with mass in LMR and, after sharp drop, flat in IMR Lack of polarization: emission from isotropic source Faster than linear scaling with Ncharged LMR: major contribution from pp annihilation IMR: naturally explained by mostly partonic radiation First evidence of low-pT  in-medium modifications in central In-In collisions f-puzzle Pb-Pb: impossible to reconcile results of mm from NA50 with KK from NA ee and KK channels measured by CERES agree with each other and NA49 In-In : mm and KK channels measured by NA60 agree (both yield and Teff ) <f>/Npart exceeds Pb-Pb value, Teff is in-between of conflicting Pb-Pb values Direct photons: no conclusive results. Excess seen by WA98 in Pb-Pb is compatible with thermal emission but the errors are very large

Dileptons and Photons at RHIC Energies
Introduction Experimental Results We know our reference … p+p data Low mass dilepton enhancement in Au+Au Direct virtual photons in Au+Au Comparison to Models Outlook Summary C4-1 Y. Akiba: “Dilepton Radiation in PHENIX” C4-3 Y. Yamaguchi: “Photons from PHENIX”

Lepton-Pair Continuum Physics
Modifications due to QCD phase transition Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation & modified heavy flavor suppression (enhancement) known sources of lepton pairs at s = 200 GeV Sources “long” after collision: p0, h, w Dalitz decays (r), w, f, J/y, y‘ decays Early in collision (hard probes): Heavy flavor production Drell Yan, direct radiation Baseline from p-p Thermal (blackbody) radiation in dileptons and photons temperature evolution Medium modifications of meson pp  r  l+l- chiral symmetry restoration Medium effects on hard probes Heavy flavor energy loss Large discovery potential also RHIC Axel Drees

Key Challenge for PHENIX: Pair Background
No background rejection  Signal/Background  1/100 in Au-Au Combinatorial background: e+ and e- from different uncorrelated source Need event mixing because of acceptance differences for e+ and e- Use like sign pairs to check event mixing Unphysical correlated background Track overlaps in detectors Not reproducible by mixed events: removed from event sample (pair cut) Correlated background: e+ and e- from same source but not “signal” “Cross” pairs  “jet” pairs Use Monte Carlo simulation and like sign data to estimate and subtract background π0 e+ e- γ X Subtractions dominate systematic uncertainties But are well under control experimentally! Axel Drees

Estimate of Expected Sources
Hadron decays: Fit p0 and p± data p+p or Au+Au For other mesons h, w, r, f, J/y etc. replace pT  mT and fit normalization to existing data where available Heavy flavor production: sc= Ncoll x 567±57±193mb from single electron measurement Hadron data follows “mT scaling” Poster by S. Milov Axel Drees Predict cocktail of known pair sources

Dilepton Continuum in p+p Collisions
Phys. Lett. B 670, 313 (2009) Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptance Data are efficiency corrected Excellent agreement of data and hadron decay contributions with 30% systematic uncertainties Axel Drees 52

Charm and Bottom Contribution
Subtract hadron decay contribution and fit difference: Phys. Lett. B 670, 313 (2009) Consistent with PHENIX single electron measurement sc= 567±57±193mb Charm: integration after cocktail subtraction sc=544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb Simultaneous fit of charm and bottom: sc=518 ± 47 (stat) ± 135 (sys) ± 190 (model) mb sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb Axel Drees 53

Contribution from Direct (pQCD) Radiation
Measuring direct photons via virtual photons: any process that radiates g will also radiate g* for m<<pT g* is “almost real” extrapolate g*  e+e- yield to m = 0  direct g yield m > mp removes 90% of hadron decay background S/B improves by factor 10: 10% direct g  100% direct g* arXiv: 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV pQCD q g hadron decay cocktail Axel Drees Small excess at for m<< pT consistent with pQCD direct photons

Au+Au Dilepton Continuum
Excess 150 <mee<750 MeV: 3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model) hadron decay cocktail tuned to AuAu Charm from PYTHIA filtered by acceptance sc= Ncoll x 567±57±193mb Charm “thermalized” filtered by acceptance sc= Ncoll x 567±57±193mb Intermediate-mass continuum: consistent with PYTHIA if charm is modified room for thermal radiation Axel Drees 55

Centrality Dependence of Low Mass Continuum
Excess region: 150 < m < 750 MeV Yield / (Npart/2) in two mass windows p0 region: production scales approximately with Npart Excess region: expect contribution from hot matter in-medium production from pp or qq annihilation yield should scale faster than Npart (and it does) Excess mostly in central AuAu yield increase faster than Npart p0 region: m < 100 MeV Axel Drees

pT Dependence of Low Mass Enhancement
arXiv: arXiv: Au+Au p+p 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c Low mass excess in Au-Au concentrated at low pT! Axel Drees

Mass Dependent Dilepton pT Spectra
p+p Au+Au detector g* e+ e- B-field Case A: g* and e+e- in acceptance Case B: g* in acceptance e+ and/or e- NOT in acceptance Apply acceptance correction correction depends on source virtual photon polarization g* different from charm additional uncertainties!! p+p consistent with cocktail up to 3 GeV/c Above mp Au+Au data enhanced for all pT most prominent for pT < 1 GeV/c Axel Drees

Local Slopes of Inclusive mT Spectra
Tdata ~ 240 MeV Tcocktail ~ 280 MeV Tdata ~ 120 MeV Tcocktal ~ 200 MeV Data have soft mT component not expected from hadron decays Note: Local slope of all sources! need more detailed analysis isolate excess (subtract cocktail) statistics limited However: low mT result will not change much! Soft component below mT ~ 500 MeV: Teff < 120MeV independent of mass more than 50% of yield Axel Drees

Dilepton Excess at High pT – Small Mass
arXiv: arXiv: Au+Au p+p 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c hadron decay cocktail 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c Significant direct photon excess beyond pQCD in Au+Au Axel Drees

Interpretation as Direct Photon
Relation between real and virtual photons: Virtual Photon excess At small mass and high pT Can be interpreted as real photon excess Extrapolate real g yield from dileptons: Example for one pT range: Excess*M (A.U). no change in shape can be extrapolated to m=0 Axel Drees

First Measurement of Thermal Radiation at RHIC
pQCD g* (e+e-)  m=0 g T ~ 220 MeV Direct photons from real photons: Measure inclusive photons Subtract p0 and h decay photons at S/B < 1:10 for pT<3 GeV Direct photons from virtual photons: Measure e+e- pairs at mp < m << pT Subtract h decays at S/B ~ 1:1 Extrapolate to mass 0 First thermal photon measurement: Tini > 220 MeV > TC Axel Drees

Comparison to Theoretical Models
A short reminder: Models for contributions from hot medium (mostly pp from hadronic phase) Vacuum spectral functions Dropping mass scenarios Broadening of spectral function Broadening of spectral functions worked well at SPS energies (CERES and NA60) pp annihilation with medium modified r works very well at SPS energies! Axel Drees

In Medium Mesons at RHIC???
Models calculations with broadening of spectral function: Rapp & vanHees Central collisions scaled to mb + PHENIX cocktail Dusling & Zahed Bratkovskaya & Cassing broadening broadening and dropping Au-Au mb with modified charm pp annihilation with medium modified r insufficient to describe RHIC data! closer look at pt spectra Axel Drees

Model Comparison in pT pp-annihilation with meson broadening
underestimates range 300 to 500 MeV maybe does ok in range 500 to 750 MeV Very different results for 810 to 990 MeV Different treatment of thermal radiation and hard contributions How about direct radiation? Axel Drees

Thermal Photon Contribution
Theory calculation by Ralf Rapp Vaccuum EM correlator Hadronic Many Body theory Dropping Mass Scenario QGP (qq annihilation only q+g  q+g* not included) q+g  q+g* ? y=0 pT ~1 GeV/c Expect virtual photon yield from QGP as large as from Hadron Gas Axel Drees Taken from talk by Y.Akiba

Calculation of Thermal Photons
Reasonable agreement with data factors of two to be worked on .. Initial temperatures and times from theoretical model fits to data: 0.15 fm/c, 590 MeV (d’Enterria et al.) 0.2 fm/c, MeV (Srivastava et al.) 0.5 fm/c, MeV (Alam et al.) 0.17 fm/c, MeV (Rasanen et al.) 0.33 fm/c, MeV (Turbide et al.) Correlation between T and t0 Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) Axel Drees

A VERY Naïve Speculation:
Extrapolate direct photons to all mass and pT Lowest order pQCD qg-Compton Add to Rapp & vanHess in-medium contribution ok for 300 to 500 MeV (close to where direct component was fitted) For higher mass yield over estimated Miss low mT contribution pQCD Maybe not that simple! Needs a theorist to calculate properly! Axel Drees

Outlook into the Future (on Tape)
Cu+Cu data finalized soon Cover low Npart range High statistics pp data (4x) Continuum between J/ and  Even better reference including bottom High statistics d+Au Cold nuclear matter effects 500 GeV data with HBD! Cu+Cu Poster by S. Campbell Poster by J. Kamin d+Au raw data Axel Drees

Future of the Dilepton Continuum at RHIC
Open experimental issues Large combinatorial background prohibits precision measurements in low mass region! Disentangle charm and thermal contribution in intermediate mass region! signal electron Cherenkov blobs partner positron needed for rejection e+ e- qpair opening angle ~ 1 m HBD Need tools to reject photon conversions and Dalitz decays and to identify open charm PHENIX  hadron blind detector (HBD) vertex tracking (VTX) HBD is fully operational Proof of principle in 2007 Taking data right now with p+p Expect large Au+Au data set in 2010 Poster by M. Durham I’m looking forward to competition from STAR once TOF is completed! Axel Drees

Summary Dilepton Data from PHENIX
background subtraction well controlled experimentally well established p+p reference discovered a low mass enhancement in central Au+Au mostly in central collisions mostly at low mT component with T~ 120 MeV independent of mass present a first measurement of thermal photons indicate initial temperature > 220 MeV Theoretical models: much work is needed! pp annihilation with collision broadening insufficient to explain data much of the radiation does not come from the hadronic phase! more complete calculation of QGP contribution needed maybe in sQGP perturbative approach not ideal! thermal radiation from QGP consistent with direct photon data initial temperature 300 to 600 MeV uncomfortably large differences between calculations! Outlook: much more data to come hopefully not only from PHENIX Axel Drees

Backup Slides Axel Drees

Search for Thermal Photons via Real Photons
The internal conversion method should also work at LHC! internal conversions PHENIX has developed different methods: Subtraction or tagging of photons detected by calorimeter Tagging photons detected by conversions, i.e. e+e- pairs Results consistent with internal conversion method Axel Drees

Combinatorial Background: Like Sign Pairs
Shape from mixed events Excellent agreements for like sign pairs also with centrality and pT Normalization of mixed pairs Small correlated background at low masses from double conversion or Dalitz+conversion normalize B++ and B- - to N++ and N- - for m > 0.7 GeV Normalize mixed + - pairs to Subtract correlated BG Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and unlike sign: 0.2 % --- Foreground: same evt N++ --- Background: mixed evt B++ Au-Au Normalization of mixed events: systematic uncertainty = 0.25% Axel Drees

Au-Au Raw Unlike-Sign Mass Spectrum
arXiv: Run with added Photon converter 2.5 x background Excellent agreement within errors! Unlike sign pairs data Mixed unlike sign pairs normalized to: Systematic errors from background subtraction: ssignal/signal = sBG/BG * BG/signal  up to 50% near 500 MeV 0.25% as large as 200!! Axel Drees

p-p Raw Data: Correlated Background
Cross pairs Simulate cross pairs with decay generator Normalize to like sign data for small mass Like Sign Data Correlated Signal = Data-Mix Jet pairs Simulate with PYTHIA Normalize to like sign data Mixed events Unlike sign pairs same simulations normalization from like sign pairs Unlike Sign Data Alternative methode Correct like sign correlated background with mixed pairs Signal: S/B  1 Axel Drees

Centrality Dependence of Background Subtraction
Compare like sign data and mixed background Evaluation in 0.2 to 1 GeV range For all centrality bins mixed event background and like sign data agree within quoted systematic errors!! Similar results for background evaluation as function pT Axel Drees

Background Description of Function of pT
Good agreement Axel Drees

Dileptons as a Probe of QGP

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Dileptons as a Probe of QGP

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Presentation on theme: "Dileptons as a Probe of QGP"— Presentation transcript:

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About project

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